This invention relates to a solid electrolyte capacitor of the type having a dielectric oxide film formed on the surface of an anode body of a valve metal and overlaid with a metal oxide electrolyte layer and a method of producing the same through pyrolysis of a metal salt solution to form the electrolyte layer.
Manganese dioxide or lead dioxide, particularly the former, is predominant as the solid electrolyte in conventional solid electrolyte capacitors of the type having an anode body of a valve metal such as tantalum or aluminum which is subjected to anodization to provide a dielectric oxide film thereon. One of primary reasons for the use of manganese dioxide being preferred is that manganese dioxide in the form of thin film can be formed relatively easily by thermal decomposition of an inexpensive manganese salt such as manganese nitrate as a solution with which the anodized valve body is wetted. As another reason, manganese dioxide has a resistivity low enough to be regarded as a semiconductive material and an oxidizing ability high enough to exhibit a so-called "healing" or "reforming" activity on the dielectric oxide film.
In a capacitor of the described type, the dielectric film formed by anodization of the valve metal body (by passing a DC current through the body in an electrolyte solution, as is well known) serves as the origin of the capacitance of the capacitor because it has a considerably high dielectric constant, 20-30 for Ta.sub.2 O.sub.5, and a very small thickness ranging from about 50 to about 2000 A. However, it is practically impossible to form this film as an ideally homogeneous dielectric layer. The dielectric oxide film has a large number of microscopical faults which cause an increase in the leakage current of the capacitor. When these faults include extremely defective ones, the capacitor is liable to break down through concentration of excessively large currents at the most significantly defective points. When the dielectric oxide film is overlaid with a manganese dioxide solid electrolyte layer, oxygen ions are supplied from manganese dioxide to the faults in the dielectric oxide layer and heal the faults, particularly extremely defective ones either as a thermal or a electrochemical phenomenon. As a result, the leakage of current in the dielectric oxide film falls to a tolerable level. Such a property of the manganese dioxide layer during operation of the capacitor is commonly referred to as "self-healing". Thus, if it is intended to use a different metal oxide in place of manganese dioxide or lead dioxide from any reason, the metal oxide is required of having a self-healing property comparable to that of manganese dioxide in order to maintain the leakage current of the capacitor at a sufficiently low level and afford the capacitor a high breakdown strength. Besides, to be low in resistivity is a requisite to a solid electrolyte for a capacitor of the herein described type since the loss expressed by tan .delta., a component of the series resistance of the capacitor, decreases as the resistivity of the electrolyte lowers.
From a consideration of either the self-healing property or electrical resistance, a solid electrolyte layer represented by a manganese dioxide layer must be formed so as to be in intimate contact with the dielectric oxide film over the entire surface area. This can be realized without much difficulty insofar as the dielectric oxide film is formed on a flat surface of the anode bobdy, but in practice the matter is not so simple. Most of currently produced tantalum capacitors of the described type utilize a sintered mass as their anode body. A sintered tantalum anode body is produced in various sizes usually ranging from about 0.01 to about 5 grams in weight, but commonly is of a porous or spongy structure having intercommunicating and microscopical voids or pores of about 5-10 .mu.m in diameter. Upon anodization, a dielectric oxide film is formed on the wall of each of these pores. A great care is demanded, therefore, in forming a manganese dioxide layer on this type of anode body so as to fully and intimately cover the complicated dioxide films in the pores.
The most prevailing method of forming a manganese dioxide layer on a sintered and anodized valve metal body consists of a wetting step, wherein the body is immersed in an aqueous manganese nitrate solution to impregnate the pores in the body with the solution, and a heating step wherein the wetted body is heated in air so as to cause pyrolysis of the impregnated manganese nitrate into manganese dioxide. It is impossible to well cover the dielectric film with a sufficiently dense manganese dioxide layer by performing these two sequential steps only once. To take out or utilize a potential capacitance given by the dielectric oxide film on the pore walls to the extent of near 100%, it is necessary to repeat the wetting and heating steps five to seven times in succession so as to fill the pores with manganese dioxide as completely as possible, each time using a relatively dilute manganese nitrate solution. Repeating the pyrolysis operation over and over raises the cost of production and, as a matter of more concern, tends to impair the dielectric oxide film by the effect of heat.
To summarize, manganese dioxide is a fairly good solid electrolyte for tantalum or aluminum capacitors but yet is not fully satisfactory in its resistivity. Besides, a widely practiced pyrolysis method for the formation of a good manganese dioxide layer on a sintered anode body is deemed too troublesome and rather difficult to perform successfully.